Commercial and industrial purchasers of paper towels and paper napkins wish to receive such products in packages that are as compact as possible. In many cases each such package is intended to provide a fill for a dispenser, and the package should contain the largest number of units that can be fitted into the dispenser. Since such a product occupies much less space when compressed than when unconfined, the product should be under substantial compression in the package.
Some prior compression packages have comprised cardboard tubes of rectangular cross-section, into which precompressed packs of product were inserted. Although these were relatively satisfactory, the cardboard tubes were also relatively expensive.
Paper bands have also been used to confine stacks of paper towels or the like under compression. Although low in cost, paper bands could sustain only limited tension and therefore did not lend themselves well to the making of an adequately compact package. Furthermore, packages comprising paper bands had a tendency to be somewhat out of straightness, or to have a rumpled edge, or in some other way to be deficient in clean-cut neatness.
Light-weight thermoplastic sheet material, which is tough, inexpensive and somewhat elastic, is very suitable to be formed into bands or sleeves for compression packages, but its employment for the purpose poses numerous problems. Some of these problems arise from the nature of that material, which is very supple and tends to be charged with static electricity that makes it sticky. Hence control of the material requires that every edge portion of it shall at all times be either supported or maintained under tension, substantially all along its length. Other problems are posed by the need for bonding such material by forming heat fused seams that have little strength until they are cooled substantially to ambient temperature.
To avoid these problems, one procedure heretofore used for obtaining a compact banded package with thermoplastic sheet material was to form the material into a band around a relatively uncompressed pack of product, and then heat-shrink the band to compress the product. Expensive energy had to be expended for the heat shrinking step, and in the resultant package the product was not necessarily compressed to the fullest or most desirable extent.
In some machines heretofore employed for compacted band packaging of compressible product, each band was formed from a web of thermoplastic sheet material that was drawn off of a braked supply roll under lengthwise tension, and the tension thus imparted to the band was relied upon to compress the product. Machines which operated on this principle tended to be slow because of the braking of the supply roll. Furthermore, the amount of compressive compaction that could be imparted to the product, which depended upon the tensioned stretching of the band material, tended to be limited.
The present invention contemplates a packaging machine whereby individual wrapper bands are formed from thermoplastic sheet material which comes to the wrapping or banding zone in continuous, substantially untensioned webs that are drawn off of unbraked supply rolls rotating continuously at steady rates. Since the band material is not substantially tensioned at the time it is looped around the product, the product must be kept under compression during the band forming step, and in fact it should then be under more compression than is desired for it in the finished package, so that it can subsequently expand into good holding engagement with the band. This poses the problem of providing holding means for maintaining the product under compression while a band is formed around it and cooperating means for so forming the band that the thermoplastic sheet material is always under control. Furthermore, the holding means and the band forming means must so cooperate that the band and the pack of product can eventually be separated from the holding means without also separating the product from the band.
These problems have been very satisfactorily solved in the machine of the present invention, but the solutions to these basic problems have brought in their train other problems of at least equal difficulty. In particular, the general arrangement and functioning of the apparatus contemplated by this invention entails a requirement for a very compact cutting and heat-bonding means whereby a cut can be made through flatwise superposed layers of thermoplastic sheet material and the layers can be substantially simultaneously heated-bonded to one another to form seams along both of the edges that result from the cut. To be completely satisfactory in a machine of the type contemplated by this invention, the cutting and heat bonding means must be capable of performing an operation quickly, must be able to repeat the operation at short intervals, but must nevertheless operate consistently even when there happen to be long intervals between successive cutting and heat-bonding operations. This is to say that the device should get hot enough to perform a cutting and heat-bonding operation in a very brief period of time, but it should nevertheless not overheat during the course of a very long delay between operating cycles.
One type of prior apparatus for performing a severing and seam-fusing function, disclosed in U.S. Pat. No. 2,686,556, to Gerber et al, comprised a high frequency generator having a frequency range on the order of 10 to 300 mega-Hertz. That apparatus may have functioned satisfactorily, but it would not be well suited to present requirements. In addition to the cost and inconvenience of providing the generator itself, its presence is now known to mandate costly and inconvenient safety precautions for protection of personnel from its high frequency radiations.
Resistance-heat cutting and heat-bonding devices were disclosed in U.S. Pat. No. 3,032,257, to Weber, and in U.S. Pat. No. 3,083,757, to Kraft et al; but in each of these a resistance heating element was embedded in a rather thick blade body, so that the device lacked the compactness and light weight that are important for the purposes of the present invention. More important, the heat that was abstracted from the blade body at each cutting and seam-fusing operation had to be replaced by conduction through the body from the embedded heating element, with the result that the device tended to have a slow cycle time; whereas if there was a long delay between successive operations the temperature of the blade body and heating element would continuously rise. Furthermore, in each of these disclosed devices the cutting portion of the blade body was on a narrow, sharp-edged, projecting portion of the body that encouraged radiation of heat, whereas the seam forming or heat-bonding portions of the body were more massive in relation to their surface areas and may well have remained hotter than the cutting portion, instead of being cooler than the cutting portion as is desired for reliable operation.